Need More Info On Isolation Transformers

[Electronic M]

Originally Posted by TinCanAlley

Okay, I understand the basics behind using one, but in my reading I've run across something I need a bit more understanding.

I keep running into not needing one if the device has a transformer. If I'm understanding this correctly, if the device gets power from a two prong and the neutral side goes to the chassis, and the hot goes through rectifiers and such, you need one. If the device has two prongs and goes directly into a transformer to create the voltages necessary for the circuits, you don't need one. Is this correct?

I'll use my Zenith as an example. It has a transformer. If one isn't necessary, I feel better about scoping a few years back. If it did need one, then I feel lucky I scoped it without issue. Is the Zenith Avante with a transformer okay to scope without one?

Same with my Pioneer SX-1080. It has a transformer. So is this an example of a device an IT wouldn't be necessary?

I really don't have anything that doesn't have a transformer, but I understand many TVs from the 60s and earlier didn't.

Thanks for the input. I want to put my Zenith back into diagnostics for the jailbars, but now am not sure it is wise without and IT.

You're mostly right.

First off IIRC your Zenith was a 25DC56 or similar which doesn't need an isolation transformer.

Can't comment on the pioneer never seen one or a schematic.

Sets without a power transformer always need an isolation transformer if you want to connect a scope.
Sets with a power transformer often don't need an isolation transformer, but exceptions exist....there were a variety of radios and TVs that had filament only transformers (my Zenith MJ1035 radio is such a set) and some sets that used a non-isolating auto-transformer (primary and secondary are directly connected together...most variacs are this but with a continuously variable tap) and series heaters (some euro radios are like this IIRC). Always a good idea to check the schematic. Also once in a while a transformer will develop a primary to secondary short...I had a 50s bench isolation transformer do that to me and it took a few zaps and big sparks connection test equipment for me to realize something wasn't right.

If one lead of the cord connects to chassis even via a switch it's a hot chassis.
Most hot chassis sets lack power cord polarization so there's only a %50 percent chance the chassis is connected to neutral. Also many hot chassis sets wire the switch such that if the chassis is at neutral when on the neutral gets opened when the set is off and the chassis floats up to hot through the tube heaters. Even if you add polarization and rewire the switch there's LOTS of reverse wired wall outlets in the world...
And even with chassis at neutral the ground and neutral voltages can be different in some circumstances and or there can be ground loop noise that can cause odd scope behavior.